The most common language targeting Java virtual machine by producing Java bytecode is Java. For local variables beyond 3 the suffix is dropped and operands must be used.įurther information: List of JVM languages istore_1 stores the integer on the top of the stack into local variable 1. The aload_0 instruction pushes the object in local variable 0 onto the stack (this is usually the this object). The n for the load and store instructions specifies the index in the local variable array to load from or store to. There is also an aconst_null, which pushes a null reference. For example, iconst_5 will push an integer (32 bit value) with the value 5 onto the stack, while dconst_1 will push a double (64 bit floating point value) with the value 1 onto the stack. The const instructions push a value of the specified type onto the stack. The const, load, and store instructions may also take a suffix of the form _ n, where n is a number from 0–3 for load and store. : 2.11.1 These are as follows:įor example, iadd will add two integers, while dadd will add two doubles. Many instructions have prefixes and/or suffixes referring to the types of operands they operate on. There are also a few instructions for a number of more specialized tasks such as exception throwing, synchronization, etc. Object creation and manipulation ( new, putfield). Instructions fall into a number of broad groups: The third is used for debuggers to implement breakpoints. : 6.2 Two of these ( impdep1 and impdep2) are to provide traps for implementation-specific software and hardware, respectively. Of the 256 possible byte-long opcodes, as of 2015, 202 are in use (~79%), 51 are reserved for future use (~20%), and 3 instructions (~1%) are permanently reserved for JVM implementations to use. : 2.6.2 Instruction set įurther information: List of Java bytecode instructionsĮach bytecode is composed of one byte that represents the opcode, along with zero or more bytes for operands. long and double types, which are 64 bits, take up two consecutive local variables : 2.6.1 (which need not be 64-bit aligned in the local variables array) or one value in the operand stack (but are counted as two units in the depth of the stack). : 4.7.3 Each can be independently sized from 0 to 65535 values, where each value is 32 bits. The maximum size of the operand stack and local variable array, computed by the compiler, is part of the attributes of each method. : 2.6 The operand stack is used for operands to computations and for receiving the return value of a called method, while local variables serve the same purpose as registers and are also used to pass method arguments. Each frame for a method call has an "operand stack" and an array of "local variables". The JVM is both a stack machine and a register machine. However, as suggested in the IBM developerWorks journal, "Understanding bytecode and what bytecode is likely to be generated by a Java compiler helps the Java programmer in the same way that knowledge of assembly helps the C or C++ programmer." Instruction set architecture In computing, Java bytecode is the bytecode-structured instruction set of the Java virtual machine (JVM), a virtual machine that enables a computer to run programs written in the Java programming language and several other programming languages, see List of JVM languages.Ī Java programmer does not need to be aware of or understand Java bytecode at all. Please consider expanding the lead to provide an accessible overview of all important aspects of the article. The first two of them are for pointers and the last one is to fill the edges and connect the pointers.This article's lead section may be too short to adequately summarize the key points. To set up our diagram correctly, we use three types of characters “├──”, “└──”, and “│” to visualize nodes.
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